title: Crystallographic orientation inhomogeneity and crystal splitting in biogenic calcite.
abstract: The calcitic prismatic units forming the outer shell of the bivalve Pinctada margaritifera have been analysed using scanning electron microscopy-electron back-scatter diffraction, transmission electron microscopy and atomic force microscopy. In the initial stages of growth, the individual prismatic units are single crystals. Their crystalline orientation is not consistent but rather changes gradually during growth. The gradients in crystallographic orientation occur mainly in a direction parallel to the long axis of the prism, i.e. perpendicular to the shell surface and do not show preferential tilting along any of the calcite lattice axes. At a certain growth stage, gradients begin to spread and diverge, implying that the prismatic units split into several crystalline domains. In this way, a branched crystal, in which the ends of the branches are independent crystalline domains, is formed. At the nanometre scale, the material is composed of slightly misoriented domains, which are separated by planes approximately perpendicular to the c-axis. Orientational gradients and splitting processes are described in biocrystals for the first time and are undoubtedly related to the high content of intracrystalline organic molecules, although the way in which these act to induce the observed crystalline patterns is a matter of future research.
authors.affiliation: Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, Granada, Spain. acheca@ugr.es
authors.forename: Antonio G
authors.initials: AG
authors.lastname: Checa
authors.forename: Jan T
authors.initials: JT
authors.lastname: Bonarski
authors.forename: Marc G
authors.initials: MG
authors.lastname: Willinger
authors.forename: Marek
authors.initials: M
authors.lastname: Faryna
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Berent
authors.forename: Bogusz
authors.initials: B
authors.lastname: Kania
authors.forename: Alicia
authors.initials: A
authors.lastname: González-Segura
authors.forename: Carlos M
authors.initials: CM
authors.lastname: Pina
authors.forename: Jan
authors.initials: J
authors.lastname: Pospiech
authors.forename: Adam
authors.initials: A
authors.lastname: Morawiec
mesh_terms: D060105:Animal Shells / Q000378:metabolism* / Q000648:ultrastructure*; D000818:Animals; D002119:Calcium Carbonate / Q000378:metabolism*; D046529:Microscopy, Electron, Transmission / Q000379:methods; D049911:Pinctada / Q000378:metabolism* / Q000648:ultrastructure*
chemical_list: D002119:Calcium Carbonate
keywords: biomineralization; bivalves; calcite; crystallography; electron back-scatter diffraction